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9th Week of 2022

Computer Science⚑

GNULinux⚑

  • New: Download only the audio in MP3.

    youtube-dl --extract-audio --audio-format mp3 [url]

demicode⚑

  • New: Get BIOS version.

    dmidecode -t bios -q

neovim⚑

  • New: Vim-surround shortcuts.

    • ysiw": Surround the word under the cursor with double quotes.
    • csiw"': Change the quotes surrounding the word under the cursor from double quotes to single quotes.
    • ds': Delete the single quotes surrounding the word under the cursor.
    • ysi)": Surround the text inside parentheses with double quotes.
    • cs)]: Change the parentheses surrounding the text inside parentheses to square brackets.

PostgreSQL⚑

  • New: Get the 20 slowest queries.

    SELECT substring(query, 1, 50) AS short_query,
              round(total_time::numeric, 2) AS total_time,
              calls,
              round(mean_time::numeric, 2) AS mean,
              round((100 * total_time / sum(total_time::numeric) OVER ())::numeric, 2) AS percentage_cpu
FROM  pg_stat_statements
ORDER BY total_time DESC
LIMIT 20;

  • New: Count NULL values.

    SELECT COUNT(*) FROM table WHERE column IS NULL;
    COUNT only conseiders non null values so use COUNT(*) (which counts the row) instead of COUNT(column) because the later would always return 0 since the filter gets only the null values of column.

Theory⚑

Algorithms⚑

  • New: Add union-find data structure definition and algorithms.

    In computer science, a disjoint-set data structure, also called a union–find data structure or merge–find set, is a data structure that stores a collection of disjoint (non-overlapping) sets.

    It can be used to store the connected components of an undirected graph.

    An example could be:

    0   1---2   3---4

5---6   7   8   9
    • N is the number of nodes. Equals to 10 in the example.

    The data structure should support the following operations:

    • initialize(): Set the initial state of the graph.
    • find(p, q): Are nodes p and q connected?
    • union(p,q): Connect nodes p and q.